71 swinger fuel issue

The stock mechanical fuel pump does create a suction on the inlet side. If there is even a slight air leak in that line, air is all you'll get. In most cases the short rubber line from fuel rail to pump is OEM with a crimped clamp. Start hunting the leak right there.
The interior of that piece of hose may be shot too so best to just replace hose and clamps here and the similar assembly at the tank. 12 inches of 5/16 neoprene hose and 4 clamps may be all you need. Good luck